We have had reports of applications running faster when executing under OMPI’s 
mpiexec versus when started by srun. Reasons aren’t entirely clear, but are 
likely related to differences in mapping/binding options (OMPI provides a very 
large range compared to srun) and optimization flags provided by mpiexec that 
are specific to OMPI.

OMPI uses PMIx for wireup support (starting with the v2.x series), which 
provides a faster startup than other PMI implementations. However, that is also 
available with Slurm starting with the 16.05 release, and some further 
PMIx-based launch optimizations were recently added to the Slurm 17.11 release. 
So I would expect that launch via srun with the latest Slurm release and PMIx 
would be faster than mpiexec - though that still leaves the faster execution 
reports to consider.

HTH
Ralph


> On Dec 18, 2017, at 2:18 PM, Prentice Bisbal <pbis...@pppl.gov> wrote:
> 
> Greeting OpenMPI users and devs!
> 
> We use OpenMPI with Slurm as our scheduler, and a user has asked me this: 
> should they use mpiexec/mpirun or srun to start their MPI jobs through Slurm?
> 
> My inclination is to use mpiexec, since that is the only method that's 
> (somewhat) defined in the MPI standard and therefore the most portable, and 
> the examples in the OpenMPI FAQ use mpirun. However, the Slurm documentation 
> on the schedmd website say to use srun with the --mpi=pmi option. (See links 
> below)
> 
> What are the pros/cons of using these two methods, other than the portability 
> issue I already mentioned? Does srun+pmi use a different method to wire up 
> the connections? Some things I read online seem to indicate that. If slurm 
> was built with PMI support, and OpenMPI was built with Slurm support, does it 
> really make any difference?
> 
> https://www.open-mpi.org/faq/?category=slurm
> https://slurm.schedmd.com/mpi_guide.html#open_mpi
> 
> 
> -- 
> Prentice
> 
> _______________________________________________
> users mailing list
> users@lists.open-mpi.org
> https://lists.open-mpi.org/mailman/listinfo/users

_______________________________________________
users mailing list
users@lists.open-mpi.org
https://lists.open-mpi.org/mailman/listinfo/users

Reply via email to